Production of a semiconductor device involves a step of forming an electroconductive film on the surface of a wafer to form a wiring layer by photolithography, etching etc., a step of forming an interlaminar insulating film on the wiring layer, etc., and an uneven surface made of an electroconductive material such as metal and an insulating material is generated on the surface of a wafer by these steps. In recent years, processing for fine wiring and multilayer wiring is advancing for the purpose of higher integration of semiconductor integrated circuits, and accordingly techniques of planarizing an uneven surface of a wafer have become important.
As the method of planarizing an uneven surface of a wafer, a CMP method is generally used. CMP is a technique wherein while the surface of a wafer to be polished is pressed against a polishing surface of a polishing pad, the surface of the wafer is polished with an abrasive in the form of slurry having abrasive grains dispersed therein (hereinafter, referred to as slurry).
As shown in FIG. 1, a polishing apparatus used generally in CMP is provided for example with a polishing platen 2 for supporting a polishing pad 1, a supporting stand (polishing head) 5 for supporting a polished material (wafer) 4, a backing material for uniformly pressurizing a wafer, and a mechanism of feeding an abrasive. The polishing pad 1 is fitted with the polishing platen 2 for example via a double-coated tape. The polishing platen 2 and the supporting stand 5 are provided with rotating shafts 6 and 7 respectively and are arranged such that the polishing pad 1 and the polished material 4, both of which are supported by them, are opposed to each other. The supporting stand 5 is provided with a pressurizing mechanism for pushing the polished material 4 against the polishing pad 1.
When such CMP is conducted, there is a problem of judging the planarity of wafer surface. That is, the point in time when desired surface properties or planar state are reached should be detected. With respect to the thickness of an oxide film, polishing speed etc., the polishing treatment of a test wafer has been conducted by periodically treating the wafer, and after the results are confirmed, a wafer serving as a product is subjected to polishing treatment.
In this method, however, the treatment time of a test wafer and the cost for the treatment are wasteful, and a test wafer and a product wafer not subjected to processing are different in polishing results due to a loading effect unique to CMP, and accurate prediction of processing results is difficult without actual processing of the product wafer.
Accordingly, there is need in recent years for a method capable of in situ detection of the point in time when desired surface properties and thickness are attained at the time of CMP processing, in order to solve the problem described above. In such detection, various methods are used. The detection means proposed at present include:
(1) a method of detecting torque wherein the coefficient of friction between a wafer and a pad is detected as a change of the rotational torque of a wafer-keeping head and a platen (U.S. Pat. No. 5,069,002),
(2) an electrostatic capacity method of detecting the thickness of an insulating film remaining on a wafer (U.S. Pat. No. 5,081,421),
(3) an optical method wherein a film thickness monitoring mechanism by a laser light is integrated in a rotating platen (JP-A 9-7985 and JP-A 9-36072),
(4) a vibrational analysis method of analyzing a frequency spectrum obtained from a vibration or acceleration sensor attached to a head or spindle,
(5) a detection method by applying a built-in differential transformer in a head,
(6) a method wherein the heat of friction between a wafer and a polishing pad and the heat of reaction between slurry and a material to be polished are measured by an infrared radiation thermometer (U.S. Pat. No. 5,196,353),
(7) a method of measuring the thickness of a polished material by measuring the transmission time of supersonic waves (JP-A 55-106769 and JP-A 7-135190), and
(8) a method of measuring the sheet resistance of a metallic film on the surface of a wafer (U.S. Pat. No. 5,559,428). At present, the method (1) is often used, but the method (3) comes to be used mainly from the viewpoint of measurement accuracy and spatial resolution in non-constant measurement.
The optical detection means as the method (3) is specifically a method of detecting the endpoint of polishing by irradiating a wafer via a polishing pad through a window (light-transmitting region) with a light beam, and monitoring an interference signal generated by reflection of the light beam.
At present, a He—Ne laser light having a wavelength light in the vicinity of 600 nm and a white light using a halogen lamp having a wavelength light in 380 to 800 nm is generally used.
In such method, the endpoint is determined by knowing an approximate depth of surface unevenness by monitoring a change in the thickness of a surface layer of a wafer. When such change in thickness becomes equal to the thickness of unevenness, the CMP process is finished. As a method of detecting the endpoint of polishing by such optical means and a polishing pad used in the method, various methods and polishing pads have been proposed.
A polishing pad having, as least a part thereof, a solid and uniform transparent polymer sheet passing a light of wavelengths of 190 to 3500 nm therethrough is disclosed (Japanese Patent Application National Publication (Laid-Open) No. 11-512977). Further, a polishing pad having a stepped transparent plug inserted into it is disclosed (JP-A 9-7985). A polishing pad having a transparent plug on the same surface as a polishing surface is disclosed (JP-A 10-83977). Further, a polishing pad wherein a light-permeable member comprises a water-insoluble matrix material and water-soluble particles dispersed in the water-insoluble matrix material and the light transmittance thereof at 400 to 800 nm is 0.1% or more is disclosed (JP-A 2002-324769 and JP-A 2002-324770). It is disclosed that a window for endpoint detection is used in any of the polishing pad.
Besides, a proposal is also offered for preventing a slurry from leaking out an interface (joint line) between a polishing region and a light-transmitting region (JP-A Nos. 2001-291686 and 2003-510826). Even in a case where each of the proposed transparent leakage preventive sheets is provided, however, the slurry is leaked out from the interface therebetween up to the lower part of a polishing layer and accumulated on the leakage preventive sheet to thereby cause a problem in optical detection of an endpoint.
A wire width of an integrated circuit will be hereinafter expected so as to be increasingly narrower in the future tendency of higher integration and leveled-up supper compactness in a semiconductor fabrication and in such a situation, a necessity arises for a high precision optical detection of an endpoint, whereas a conventional detection window for an endpoint has had a problem of the slurry leakage insufficiently solved. A conventional detection window for an endpoint have not reached a detection precision at a sufficiently satisfactory level because of limitation on usable materials. In a case where a polishing pad having a light-transmitting region was employed, problems arose that polishing characteristics (such as in-plane uniformity) were deteriorated or that scratches occurred on a wafer.
On the other hand, in performing a CMP process, a problem has arisen that a wafer is metal contaminated. In the CMP process, a wafer is polished while a slurry is caused to flow over a polishing pad and in this situation, metals contained in the slurry or the polishing pad remain on a polished wafer. Such metal contamination on a wafer results in reduction in reliability and generation of a leakage current in an insulating film and abnormality in a formed film, which exerts a great adverse influence on a semiconductor device to in turn, decreases a production yield. Especially, in the current semiconductor fabrication, when a shallow trench isolation (STI) is adopted wherein isolation on a semiconductor substrate has been a technical main stream, metal contamination in an oxide film after polishing is a very severe problem. In the STI, predetermined shallow trenches are formed on a silicon wafer and the trenches are buried with SiO2 film deposited therein. Thereafter, the surface is polished to fabricate regions isolated by the oxide film. Elements (such as transistors) are fabricated in the isolated regions; therefore, the metal contamination on the wafer surface after polishing reduces performances and lowers reliability of all the elements. Therefore, in the current technology, a wafer cleaning step has been adopted after CMP in order to decrease metal contamination on a wafer.
Cleaning a wafer, however, has many of demerits such as oxidation of wiring; therefore a desire has been piled up for less of contamination from a slurry or a polishing pad. Especially, a metal ion such as a Fe ion is hard to be removed by cleaning and easy to remain on a wafer.
Therefore, in order to solve the above problem, a proposal has been lately offered of a polishing sheet having a polishing layer of high molecular weight polyethylene based resin with a metal impurity concentration of 100 ppm or less (JP-A No. 2000-343411). Besides, another proposal has been offered of a polishing cloth for a semiconductor wafer with a zinc content of 200 ppm or less (WO 01/15860 A1)
At the above metal impurity concentration, metal contamination of a wafer cannot be sufficiently prevented, which imposes a burden in cleaning of a wafer after CMP, having lead to difficulty improving a product yield of a device.
Still another proposal has been offered of a polishing pad using an organic intermolecular crosslinking agent in which a metal atom is reduced to the lowest possible level (JP-A No. 2001-308045)
However, a concrete metal concentration contained in a polishing pad has not clearly described in the last proposal. In addition, the polishing pad is molded in a metal mold in manufacture; therefore, the polishing pad cannot decrease at all a metal contamination on a surface of a wafer.